It is well known that brittle materials cannot be simultaneously dense (which is synonymous with corrosion resistance) and thermal shock resistant. For example, if samples of a dense alumina refractory are heated in a furnace to progressively higher temperatures and then quenched in water to room temperature, and the mechanical strength of the quenched samples is measured, it will be found that at a critical temperature (which varies according to the size of the sample and the conditions under which the experiment is conducted), there is a sudden and significant reduction in the strength retained by the sample after the quenching. In one reported instance, experiments with samples of a dense alumina showed that:
(a) after quenching the sample following heating in a furnace to temperatures of up to (typically) 150.degree. C., the modulus of rupture of the quenched sample remains high (about 230 MPa in a test run conducted by the present inventor); that is, no damage to the sample has resulted from the quenching; PA0 (b) with samples quenched after heating to a furnace temperature of about 150.degree. C., the modulus of rupture of the quenched sample drops dramatically (to about 60 MPa in the test run), showing that the thermal stresses generated during the quench are sufficient to activate pre-existing surface flaws (cracks) in the alumina refractory, so that these flaws propagate catastrophically through the material, causing the sudden loss of strength of the quenched sample; and PA0 (c) when the sample is quenched from temperatures in excess of about 240 .degree.the strength of the quenched samples falls approximately exponentially, the modulus of rupture reaching a value of about 19 MPa in the test run when the furnace temperature was 400.degree. C. PA0 (a) occupy from 1.0 percent by volume to 40 percent by volume of the composite refractory material; PA0 (b) each comprise a polycrystalline agglomerate of microcrystals with no matrix material within the agglomerate, the microcrystals (i) being strongly bonded together, (ii) exhibiting a strong thermal expansion anisotropy, and (iii) having a size such that cracks do not form spontaneously within the agglomerate during cooling from a temperature of about 1600.degree. C. to room temperature; and PA0 (c) each have a mean diameter of from 10 to 15 micrometers, with the microcrystals therein having a mean diameter in the range of from 1 to 2 micrometers. PA0 (a) a matrix of a refractory material; PA0 (b) particles of a ceramic material dispersed in the matrix, the particles occupying from 1.0 percent by volume to 40 percent by volume of the composite refractory material, each particle comprising a polycrystalline agglomerate of microcrystals with no matrix material within the agglomerate, the microcrystals (i) being strongly bonded together, (ii) exhibiting a strong thermal expansion anisotropy, and (iii) having a size such that cracks do not form spontaneously within the polycrystalline agglomerate during cooling from a temperature of about 1600.degree. C. to room temperature; the ceramic material and the refractory material of the matrix being mutually chemically inert in the temperature range of the use of the composite refractory material. PA0 (a) mixing together a powder of the matrix alumina material, a powder of the polycrystalline ceramic material to be dispersed within the matrix material, and a fugitive binder; PA0 (b) allowing the mixture to dry; PA0 (c) granulating the dried powder mixture; PA0 (d) preforming the granulated powder into billets by die pressing; PA0 (e) isostatically pressing the die pressed billets to form green billets for sintering; PA0 (f) heating the green billets at a predetermined rate until a sintering temperature in the range from 1200.degree. C. to 1800.degree. C. is reached; PA0 (g) holding the green billets at the sintering temperature for a period in the range from 0 to 5 hours; and PA0 (h) furnace cooling the sintered billets to ambient temperature.
To overcome the problem of unstable crack propagation in the quenching process, the traditional approach has been to introduce porosity into the refractory material. This reducation in density also reduces the low temperature strength of the refractory but the effect of quenching from higher temperatures is less dramatic. For example, samples of the same alumina material that has been described above which have a 5 percent porosity had an inherent low temperature strength of about half that of the dense material, the low temperature modulus of rupture being approximately 103 MPa. At the critical temperature of 150.degree. C., quenching reduced the strength of the material with 5 percent porosity, but the modulus of rupture of the quenched material was about 87 MPa. Quenching samples of this material from a temperature of 400.degree. C. produced samples having a modulus of rupture of about 70 MPa.
A refractory brick which is high in alumina content and has a porosity in the range from 15 percent to 25 percent completely solves the thermal shock problem. The modulus of rupture of quenched samples of such a material varies substantially linearly from about 19 MPa when the material is quenched from low temperatures to about 17 MPa when it is quenched from a temperature of about 400.degree. C. The small loss in strength when the material is quenched from higher temperatures is because there has been stable crack propagation, but the mechanical strength at low temperatures has been sacrificed, being only about one tenth of the strength of a dense commercial alumina ceramic material. An even more serious sacrifice has also occurred, for in the case of a porous alumina refractory brick, the rate of slag erosion (corrosion) of the alumina increases exponentially with the increase in porosity.
Another illustration of the problems that can be encountered when dense ceramic materials are subjected to thermal shock is found in ceramic capacitors made from barium titanate (BaTiO.sub.3). These capacitors are important components of modern integrated circuit boards, but they are prone to failure as a consequence of thermal shock when metal electrical contacts are soldered onto the capacitors.
All ceramics contain flaws generated during manufacturing and handling which render them prone to failure by unstable crack propagation when they are subject to tensile stresses. In the case of multilayer ceramic oxide capacitors, even if only 0.5 percent of such capacitors fail, as components of integrated circuit boards, the annual economic loss in the USA is about $20,000,000. The topic of reliability of multilayer ceramic capacitors is discussed in detail in the Journal of the American Ceramic Society, volume 72, number 12, 1989, pages 2221-2294.